1. Field of the Invention
The present invention relates to a sheet processing apparatus connected to an image forming apparatus for collecting, aligning and post-processing the sheets discharged from the image forming apparatus, and more particularly to an apparatus having a stacking tray for stacking and aligning a plurality of sheets temporarily.
2. Description of the Related Art
Hitherto, the sheet processing apparatus is connected to an image forming apparatus, for conveying the sheets discharged from the image forming apparatus, aligning and sewing, and moving the sewn sheet bundle to a stacking tray. At this time, while processing the sheet bundle, next sheet bundle cannot be sent into the stacking tray, and the throughput is lowered. To solve the problem of throughput decline, it has been attempted to produce a post-processing time by temporarily superimposing and stacking two sheets before the nip section of the roller for discharging the sheets to the sheet stacking tray, before discharging sheets to the sheet stacking tray, and discharging two sheets in superimposed state onto the stacking tray, thereby preventing decline of throughput.
However, as shown in Japanese Patent Application Laid-Open (JP-A) No. 9-235069, in the configuration designed to discharge sheets to a stacking tray, after superimposing while aligning the ends of two sheets by hitting the sheets conveyed from above to below against the roller for discharging sheets to the stacking tray, since the end of sheet abuts against the discharge roller as sheet end stopper by the own weight, a wide space is needed for temporarily stacking the sheet in the conveyance path up to the discharge roller (a longer conveyance path than the maximum sheet size for post-processing is needed), and the apparatus is larger in size, and is higher in cost.
Or as disclosed in JP-A No. 1-127556, by stopping a first sheet conveyance roller, a first sheet is stopped before stacking tray discharge roller, and when the end of second sheet reaches before discharge roller pair by second sheet conveyance roller, by driving the first sheet conveyance roller, two sheets are conveyed simultaneously, and two sheets in superimposed state are discharged onto stacking tray, and even in such configuration, enough space for completely holding the first sheet within the conveyance path is required, and the apparatus is larger in size and higher in cost.
In the configurations disclosed in JP-A No. 9-235069 and in JP-A No. 1-127556, by stopping second sheet once when superimposing two sheets, the paper space to third sheet (the time until third sheet is stacked on stacking tray) is shortened, and enough aligning process time may not be produced.